Added +: and :+ extractors to mirror append/prepend.

jsuereth · jsuereth · commit b3efb3d49360 · 2012-03-16T20:59:38.000-04:00
* +: does head/tail decomposition on any Seq * :+ does init/last decomposition on any Seq * Both preserve specific Seq types. Review by @odersky
diff --git a/src/library/scala/collection/SeqExtractors.scala b/src/library/scala/collection/SeqExtractors.scala
@@ -0,0 +1,21 @@
+package scala.collection
+
+/** An extractor used to head/tail deconstruct sequences. */
+object +: {
+  def unapply[T,Coll <: SeqLike[T, Coll]](
+      t: Coll with SeqLike[T, Coll]): Option[(T, Coll)] =
+    if(t.isEmpty) None
+    else Some(t.head -> t.tail)
+}
+
+/** An extractor used to init/last deconstruct sequences. */
+object :+ {
+  /** Splits a sequence into init :+ tail.
+   * @returns Some(init, tail) if sequence is non-empty.
+   *          None otherwise.
+   */
+  def unapply[T,Coll <: SeqLike[T, Coll]](
+      t: Coll with SeqLike[T, Coll]): Option[(Coll, T)] =
+    if(t.isEmpty) None
+    else Some(t.init -> t.last)
+}
diff --git a/src/library/scala/package.scala b/src/library/scala/package.scala
@@ -64,6 +64,9 @@ package object scala {
   type ::[A] = scala.collection.immutable.::[A]
   val :: = scala.collection.immutable.::
 
+  val +: = scala.collection.+:
+  val :+ = scala.collection.:+
+
   type Stream[+A] = scala.collection.immutable.Stream[A]
   val Stream = scala.collection.immutable.Stream
   val #:: = scala.collection.immutable.Stream.#::
diff --git a/test/files/run/matchonseq.check b/test/files/run/matchonseq.check
@@ -0,0 +1,2 @@
+It worked! head=1
+It worked! last=3
diff --git a/test/files/run/matchonseq.scala b/test/files/run/matchonseq.scala
@@ -0,0 +1,8 @@
+object Test extends App{
+  Vector(1,2,3) match { 
+   case head +: tail => println("It worked! head=" + head) 
+  }
+  Vector(1,2,3) match { 
+   case init :+ last => println("It worked! last=" + last) 
+  }
+}

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+It worked! head=1`
	`2`	`+It worked! last=3`